A feature of a MEMS (Micro Electro Mechanical Systems) device lies in that it has a mechanically movable portion, unlike a normal semiconductor device. Therefore, packaging a MEMS device requires a cavity serving as the operation space of the mechanically movable portion. That is, a packaging technique/package for forming a hollow structure is used in the MEMS device.
On the other hand, for miniaturization and cost reduction, a packaging technique of forming a hollow structure in a wafer state, instead of individual packaging for each chip, has been proposed in recent years. This packaging technique is called a WLP (Wafer Level Package). Alternatively, this package can be fabricated in a clean room, and is therefore also called an in-line WLP, thin-film cap, or thin-film dome structure.
There is provided a method of manufacturing a MEMS element having a movable portion, and a thin-film dome structure which stores the MEMS element, that includes the step of forming and patterning a first sacrificing layer, the step of forming a MEMS element on the first sacrificing layer, the step of forming a second sacrificing layer on the first sacrificing layer and MEMS element, and patterning it, the step of forming a first layer on the second sacrificing layer, and the step of removing the first sacrificing layer and second sacrificing layer. The first layer partially forms the thin-film dome structure, and has through holes. An undoped silicon oxide film which has a thickness of about 1 μm and is deposited by the CVD (Chemical Vapor Deposition) method, for example, is used as the first layer.
However, when a crack is locally generated in the thin-film dome structure, it becomes the starting point of a path which connects the interior and exterior of the thin-film dome to each other. This may degrade the sealing performance of the dome interior, thus degrading the reliability of the device.